1. Technical Field
The present invention relates to an electronic structure, and associated method of formation, in which a plated metallic layer of a plated through hole (PTH) is adhesively coupled to a resin distributed within the PTH.
2. Related Art
A printed wiring board (PWB), such as a chip carrier, typically includes a substrate with a plated through hole (PTH), wherein the PTH serves to provide electrical coupling between circuitized layers of the PWB. Fabrication of a PWB includes drilling a through hole through a substrate, plating the hole with a metallic substance such as copper to form the PTH, filling the PTH with a volume of nonconductive material (called a "holefill") which adheres to the plated wall of the PTH, forming a first circuit pattern on a first PWB layer and a second circuit pattern on a second PWB layer such that the PTH electrically couples the first circuit pattern to the second circuit pattern. A circuit pattern is typically formed by photolithography, which comprises forming a metallic layer such as a copper layer on a surface of the PWB, forming a photomask layer on the metallic layer, selectively exposing portions of the photomask layer to a suitable wavelength of light (e.g., ultraviolet light) by use of a mask pattern, developing away a portion of the photomask in accordance with the mask pattern, chemically etching portions of the metallic layer not covered by protective photomask material, and removing the remaining photomask material to expose the formed circuit pattern.
The holefill, which includes a resin (or binder) such an epoxy composition, serves several purposes including providing mechanical support for structures on either end, or both ends, of the PTH. If the PWB experiences significant temperature fluctuations during PWB processing or during its operating lifetime, the holefill should be selected to have a coefficient of thermal expansion (CTE) that closely matches the CTE of the substrate material. A material mismatch in CTE would cause differential thermal expansion that could subject high-stress locations near the PTH to fatigue failure from thermal cycling. Inasmuch as holefill epoxy resin typically has a materially higher CTE than that of the substrate, the relatively higher rate of thermal expansion of the holefill may cause the holefill material to act as a piston causing damage to objects mounted on a surface of the PWB near the PTH. To mitigate this adverse effect, the holefill may include a particulate component such as copper powder whose CTE is close to that of the substrate material. In the absence of thermal cycling, the particulate component of the holefill may be unnecessary.
Most importantly, the holefill is intended to protect the PTH metallic plating from the chemical etchant used to form a circuit pattern on a surface of the PWB, as well as from other processing chemicals that could corrosively attack the PTH metallic plating. As to insulation from chemical etchants, this protective feature of the holefill is particularly relevant to the formation of thin (e.g., 1 to 2 mils) circuit lines as opposed to the more conventional thick (e.g., 2 to 4 mil) circuit lines. Forming a thick circuit pattern requires a small amount of etching of the metallic layer on the PWB surface and a consequent small quantity of etchant and as well as a small etchant exposure time. Thus even if the PTH metallic plating were unprotected from the etchant, only a small amount of PTH metallic plating would be etched away, leaving a residual PTH metallic plating that may be sufficiently thick to be electrically effective and mechanically stable during the lifetime of the PWB. In contrast, forming a thin circuit pattern requires relatively large amount of etching of the metallic layer on the PWB surface and a corresponding substantial exposure of an unprotected PTH metallic plating to the etchant, such that the PTH metallic plating may be subject to unacceptable degradation. Thus, it is important that the holefill protect the PTH metallic plating from chemical attack of etchant, or other processing chemical, by reliably maintaining its adhesive contact with the PTH metallic plating.
Unfortunately, the holefill material is generally brittle and is subject to fracture, resulting in some PTH holefill material being pulled out of the PTH during subsequent mechanical or chemical processing. The consequent void volumes within the holefill left by the removed holefill material are referred to a depressions, pits, and chip-outs. A particular cause of the void volumes in the holefill relates to the mask used during the fill operation. If a selective PTH fill is desired, a mask with a pre-drilled pattern is placed on the PWB between the holefill material and the panel of the PWB. During the filling process, this mask undergoes some mechanical coupling to the panel and the filled hole. The mask is removed from the panel after the filling process. The mechanical coupling is very weak but, in some instances, strong enough to cause a fracture of a portion of the holefill material in the PTH that extends below the plane of the panel.
Another cause of void formation in the holefill relates to grinding removal of resin "bleed." The holefill in the PTH may have a protrusion, or a "nub," that extends out of the PTH volume and slightly above the surface of the PWB. During the fill process, when a selective mask is used, some of the resin from the holefill material may migrate, or "bleed," away from the area being filled into the space between the mask and the PWB surface. This effect often occurs because of inherent imperfections in the planarity of the mask and the PWB so that even if the filling is done under vacuum lamination conditions, there may not be a tight seal between the mask and the PWB. The "bleed" can also occur during a non-selective filling. The resulting migrated material, or "resin bleed," is pasted to the surface of the surface of the PWB. The resin bleed material or the protruding "nub" may be removed by grinding with a sanding instrument, which may cause fracture of holefill material in the PTH. Thus, the propensity of holefill material to be fractured is a reliability concern, since the consequent formation of void volumes within the PTH leaves the PTH metallic plating exposed to corrosive attack by chemical etchants and other processing chemicals, as discussed supra.
The PTH metallic plating needs to be protected from chemical attack by chemical etchants and other processing chemicals should PTH holefill material be fractured and pulled away from the PTH metallic plating.